#include<bits/stdc++.h>

using namespace std;
typedef long long ll;
#define endl '\n'
const int maxn = 100 + 50;
int degree[maxn];
int n, m, from, to;

bool is_Euler_graph(int from, int to) {
    int odd = 0;
    for (int i = 1; i <= n; i++) {
        if (degree[i] & 1)++odd;
    }
    if (odd > 2)return false;
    if (from != to) {
        return odd == 2 && (degree[from] & 1) && (degree[to] & 1);
    } else {
        return odd == 0;
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    ifstream fin("C.in");
    if (fin.good()) cin.rdbuf(fin.rdbuf());
    while (cin >> n >> m) {
        cin >> from >> to;
        while (m--) {
            int x, y;
            cin >> x >> y;
            degree[x]++;
            degree[y]++;
        }
        copy(degree + 1, degree + n + 1, ostream_iterator<int>(cerr, " "));
        cerr << endl;
        cout << (is_Euler_graph(from, to) ? "YES" : "NO") << endl;
    }

    return 0;
}